It will be appreciated that ANC is a term of art, and its use herein is not intended to imply that perfect cancellation of ambient noise is achieved; merely that the levels of ambient noise as perceived by a listener can be substantially reduced by the use of ANC systems.
ANC enables the perceived loudness of the noise surrounding a user to be reduced by creating a signal that, when played through a speaker proximal to the ear of the user, produces an acoustical output that interferes destructively at the user's eardrum with the noise surrounding the user. The signal that is played through the speaker is usually created by deriving a signal representative of the ambient noise using a microphone proximal to the ear of the user and applying a filter to that signal.
In order for the system to be effective, the amplitude and phase of the filter must be correct simultaneously. A related requirement for destructive interference is that the generated signal that is played through the speaker must arrive at the user's eardrum at the same time as the ambient noise signal that was detected by the microphone and thus gave rise to the generated signal. For this to occur, the generated signal must be constructed within the time it takes for the ambient noise wave-front to propagate a distance equivalent to the distance from the sensing microphone to the speaker proximal to the ear of the user. For a typical sized circumaural noise-cancellation headphone this distance is typically about 15 mm, corresponding to a time delay of approximately 44 μs. This has specific consequences when digital processing is used because of the inherent time-delays in the analogue-to-digital and digital-to-analogue converters (briefly “ADC” and “DAC” respectively) and clocked digital signal-processing apparatus.
There is a large body of prior-art which describes digital noise-cancellation circuits. Examples include GB-A-2149614 in which the fundamental frequencies and harmonics of the ambient noise are identified and a microprocessor is used to generate an anti-noise signal; U.S. Pat. No. 6,278,786 which describes a feedback noise-cancellation system in a headset, for use in an aircraft, incorporating a hybrid analogue and digital apparatus, and a publication: “The implementation of digital filters using a modified Widrow-Hoff algorithm for the adaptive cancellation of acoustic noise” (Acoustics, Speech, and Signal-processing, IEEE International Conference on ICASSP '84, March 1984, pp. 215-218) which describes a noise-cancellation system using and “electronic controller” implementing a digital filter.
The generic steps in the signal-processing for the prior-art involve converting a signal indicative of the ambient noise to a digital form using an analogue-to-digital converter, applying a fixed filter or an adaptive filter to the digital signal, then converting the result back to analogue using a digital-to-analogue converter before sending it to a speaker located near the ear of the listener.
The most significant practical difficulty associated with using a digital processing system in a low-cost and low-power active noise-cancellation application is the selection of the ADC and DAC, because commercially available low-cost, low-noise, audio-bandwidth components tend to have a group delay in the region of 50 μs to 100 μs, i.e. in excess of the 44 μs or so needed for the present application. Examples include Analog Devices AD1974, Texas Instruments PCM3002 and Cirrus Logic CS42526.
One obvious method of decreasing the time delay incurred by the digital processing is to increase the rate at which the analogue input is sampled. This can be achieved using a high performance ADC, DAC and digital processor, but it has the disadvantage of increased cost and significantly increased electrical power consumption. This latter issue assumes particular significance when it is noted that most ANC devices are hosted by battery-powered appliances.
ADCs and DACs that use a sigma-delta modulator have been the preferred choice for audio applications over the last two decades because they can achieve very high signal resolution using a low-cost complementary metal-oxide semiconductor (CMOS) manufacturing process. Sigma-delta modulation is based on the technique of oversampling the input analogue signal, combined with noise-shaping to reduce the noise in the band of interest. The output of a sigma-delta modulator is typically a stream of N-bit digital values at a sample-rate R, where N is often 1 and usually lower than 8, and where R is often 64 times the Nyquist frequency of the input analogue signal. Audio-bandwidth sigma-delta ADCs apply additional processing to the sigma-delta bit stream to increase its precision and decrease the sample-rate.
The precision of the bit stream is increased by averaging, usually by applying a low-pass filter. A second processing step is to reduce the sample-rate using a decimator. The low-pass filter and the decimator are usually designed together as a down-sampler, where the low-pass filter is used to attenuate frequencies which would otherwise cause aliasing artefacts. Unfortunately the low-pass filter introduces a time delay which is undesirable in a digital noise-cancellation apparatus.
Much of the prior-art uses low-cost sigma-delta analogue-to-digital converters. An example of prior-art is described in “Microprocessors and Microsystems” Volume 22 (7), 25 Jan. 1999, pp. 413-422, in which an Analog Devices AD1847 sigma-delta ADC and an Analog Devices ADSP2181 fixed point DSP are used, where the author implements an adaptive FIR filter with 100 taps.
One approach to time-delay reduction is described in US-A-2009/0046867, which suggests that the time delay in a traditional sigma-delta ADC can be reduced by dispensing with the down-sampler that is traditionally found in these components, and processing the immediate output of the sigma-delta modulator. The drawback with this is that the digital processor that carries out the ANC filtering must operate at a very high sample-rate, and consequently the power consumption is high. In contrast to this, it is estimated that the power consumption of the present invention would be 75% less than that particular method.
It is an object of the present invention to provide an economical ANC device with reasonable power consumption and a processing time delay that is concomitant with an ability to efficiently implement ambient noise reduction.